A ferrofluid is a colloidal suspension of sub-domain magnetic particles in a liquid carrier. The magnetic particles are typically magnetite particles (Fe.sub.3 O.sub.4) and the like. The magnetic particles are suspended in extremely finely divided form with the help of dispersing aids, anti-agglomeration agents, and the like. The liquid carrier may be aqueous or nonaqueous.
Ferrofluids have many uses, including magnetic domain detection, magnetic adhesives, magnetic paints, lubricants, sealants, ink jet printing, and laser systems. For certain applications, e.g., magnetic domain detection, water is a more desirable liquid carrier than nonaqueous materials. Many aqueous ferrofluids have been proposed. See, e.g., U.S. Pat. Nos. 3,990,981; 4,026,713; and 4,107,063.
There are a number of concerns, however, associated with aqueous ferrofluids. One concern relates to the stability of aqueous ferrofluids. In previously known aqueous ferrofluids, the suspended magnetic particles may tend to precipitate in solution over time. Such precipitation changes the active composition of the ferrofluid. Such ferrofluids also require periodic filtering to remove the precipitate. It would be desirable to prepare an aqueous ferrofluid with improved stability to avoid such unwanted precipitation.
Another concern relates to the use of aqueous ferrofluids for magnetic domain detection. With previously known aqueous ferrofluids, individual bits, individual bytes, bit patterns, magnetic tracks and their straightness, and the like of magnetic recording media cannot be observed with high resolution or good contrast. With the trend towards higher density magnetic recording materials, an aqueous ferrofluid which would allow the magnetic structure and flux patterns of such higher density magnetic recording media to be seen with improved resolution and contrast would be highly desirable.